In data processing systems, magnetic disc memory systems are frequently used because they have large storage capacity and require a relatively short time for a magnetic read/write head to access data contained anywhere on discs of the memory from the moment the head receives an instruction to access the data. The magnetic discs are driven at constant rotational velocity by an electric motor.
A magnetic disc carries data, usually in binary form, on both faces of the disc in concentric circular recording tracks having a width that does not exceed a few hundredths of a millimeter. The tracks are identified by alloting them an address or serial number j (j being a whole number) from 0 to (N-1), where N is the total number of recording tracks. The tracks having address (j-1) and (j+1) are adjacent track j.
Memories having a small storage capacity contain a limited number of discs (normally only one or two). In small memories, data are stored, i.e., recorded, on each of the disc faces by setting a large amount of space aside for the data intended to be processed by the data processing system of which the memories are a part. A small amount of space is set aside for data that enables the tracks to be located; these data indicate the track addresses and enable the magnetic head to be servo-controlled to a position above the tracks. In the small area are also stored data which indicate whether or not the tracks contain faults and preamble data that enable the gain of the amplifier circuits responsive to the head to be controlled.
For the sake of simplicity, a memory is considered which contains only a single disc. Preferably, each face of the disc is associated with a single magnetic read/write head, i.e., a magnetic read/write transducer. Current practice, as described in commonly assigned United States Patent Application Ser. No. 76,332, filed on Sept. 17, 1979, entitled "Method of Writing Information On a Magnetic Recording Medium", is for the data contained on each face of the disc to be distributed over equal adjacent circular sectors S.sub.0, S.sub.1 . . . S.sub.i . . . S.sub.n. Generally, one face of a disc is divided into several tens of sectors (usually forty to fifty).
When the magnetic disc face associated with the magnetic head passes in front of the head, sectors S.sub.0, S.sub.1, S.sub.2, etcetera, are read by the head in sequence. It is therefore said that sector S.sub.0 precedes sector S.sub.1, that sector S.sub.1 precedes sector S.sub.2, that sector S.sub.i precedes sector S.sub.i+1 and so on. In more general terms, if two items of information I.sub.k-1 and I.sub.k which follow one another along the same track j on the face are considered, item I.sub.k-1 precedes item I.sub.k if item I.sub.k-1 is read by the head before item I.sub.k, or that item I.sub.k follows item I.sub.k-1. The same reasoning is applied to groups of information items G.sub.k and G.sub.k-1 in a track (j+1) adjacent and abutting with track j.
Each sector S.sub.i is divided into two unequal areas. The larger area contains the data to be processed by the data processing system of which the disc memory is a part, while the smaller area contains data for locating the tracks, preamble data for controlling amplifier gain and data for indicating faults. In each sector, the smaller area is divided into a plurality of reference zones, one for each track, so each track is associated with a single reference zone.
It is recalled that a bit is a binary one or zero digit. The one or zero may be expressed on a magnetic medium or as an analog or logic electrical signal. A logic signal is capable of assuming only two values called "logic or binary zero" and "logic or binary one"; an analog signal is a signal having a voltage that may vary continuously between two positive and/or negative extreme values. Any item of data or information recorded on the disc is referred to herein as a "bit".
A magnetic head for writing information into and reading information from a magnetic disc includes a magnetic circuit comprising a high magnetic permeability material on which is mounted a winding and in which is formed an air gap. The air gap is substantially rectangular in shape, having a length much greater than its width. The gap is of the same order of magnitude as the radial width of the tracks and reference zones, which are of the same width. Thereby, the gap is responsive to magnetic flux variations representing data to be processed from a disc track having serial number j, as well as track identifying data contained in reference zones ZRP.sub.ij and ZRP.sub.i(j+1) associated with the data track having serial number j. The air gap of the head is disposed perpendicularly to magnetic axis Ax.sub.j of track j, i.e., the air gap is disposed parallel to the radial width of track j. To enable the data of track j to be read from the disc or written into the disc with maximum accuracy, the head remains stationary facing the track during the time necessary for reading or writing all or part of the data which the track contains while the disc rotates at constant velocity. The head air gap is perfectly centered on magnetic axis Ax.sub.j, the boundary reference zones ZRP.sub.ij and ZRP.sub.i(j+1). The magnetic read/write head reads or writes track identifying data in reference zones ZRP.sub.ij and ZRP.sub.i(j+1) by being disposed astride the magnetic axis separating the two reference zones.
The data read by a head associated with a particular magnetic disc surface are supplied to an amplifier circuit prior to being coupled to additional processing circuitry. Data to be processed, read from the data zones, are supplied to electronic read circuits of the disc memory. The read circuits respond to these data to determine the binary values of data bits in the data zones. In contrast, data from the reference zone are supplied to circuitry for performing various control functions. For example, amplified signals derived from the amplifier in response to data read from the head correspond to addresses. The address signals are applied to a device for displacing the head with respect to the disc surface, as described in copending, commonly assigned United States Patent Application entitled "Apparatus And Method For Displacing A Movable System With Respect To A Data Carrier", Ser. No. 186,294, filed Sept. 11, 1980. In response to the address signals, the head is radially displaced from track A, where it was initially located, to track B where all or part of the data to be processed in the data zone is to be read. The amplifier also responds to the head, while in the reference zone, to derive signals indicative of the head position with respect to an axis of a track between a pair of abutting reference zones. These signals, indicative of transverse head position, are supplied to a device for controlling the position of the head so that it is desirably driven to straddle the axis between a pair of reference zones. A preferred device for controlling the head so it straddles a pair of reference zones is described in the copending, commonly assigned United States Application entitled "Apparatus For And Method Of Determining Transverse Position Of a Transducer Relative To Read Data Tracks", Ser. No. 205,863, filed Nov. 10, 1980.
It is desirable for the "data to be processed" read from the data zone of a track, which is to be read in whole or in part, to be read by the head as rapidly as possible and with maximum accuracy. In other words, it is desirable for the head output to have a miminum ratio of faults for the data to be processed. The ratio of faults is defined as the ratio between the number of data items having an incorrect value, as detected by a read circuit, and the total number of detected data items. It is desirable for the minimum ratio of faults to be considerably lower than 10.sup.-9. To achieve such a low ratio of faults, the period to displace the head from track A to track B should be as short as possible and the head gap should be centered above track B with as great an accuracy as possible, i.e., the head gap should be held perfectly astride the axis of track B. To assure this accuracy, it is also necessary for the read circuit to detect the value of the data to be processed with maximum accuracy.
To achieve the minimum ratio of faults, it is necessary for the amplitude of the analog signals derived by the amplifier circuits to be substantially constant at a sufficiently high amplitude. It is difficult to maintain the amplitude of the signals constant because the head output signal amplitude varies as a function of head radial position, assuming identical reading conditions at the different radial positions; identical reading conditions are considered to be conditions which result in the gap and disc to be separated by each other by the same amount, and the same relative position of the gap traversely of the data track. In particular, the amplitude of the signal derived from the head is greater when the head is aligned on a track situated at the periphery of the disc compared to when the head is aligned on a track adjacent the center of the disc. This is because the disc travels at a greater transverse speed relative to the head when the disc is positioned close to the periphery than when the head is positioned close to the disc center.
The output signal of the head is also a function of the distance between the head and the disc and faults which occur on the disc due to microscopic variations of evenness and flatness of the disc. In particular, dimensions of the order of one micron cause variations in the amplitude of the head output. Thereby, the amplitude of signals derived by the head corresponding to magnetic transitions forming data on the track in question may vary appreciably from one magnetic transition to another. Variations in the output of the amplifier also occur because a disc memory includes plural heads and plural discs, such that one head is associated with each disc surface in a typical situation. The amplitude of the signals derived from the different heads varies even under strictly identical reading conditions, i.e., with the same track serial number, same relative position of head gap with respect to data, same distance between the gap and disc surface associated with the head, and absence of faults of the magnetic recording layer.
Because of the variations of the amplitude of the signals derived by the head or heads as a function of these various factors, it is necessary for the gain of the amplifier circuits to be controlled. Otherwise, the desired minimum fault ratio is not achieved. The present practice is to control the gain of the amplifier circuits by signals corresponding to the preamble data recorded in a part of each reference zone ZRP.sub.ij.
One known method of recording data on the face of a magnetic disc involves providing a succession of elementary areas of variable length over the entire length of each track and each zone by applying magnetic fluxes to the zones by the magnetic head. Alternate areas have magnetic inductions of the same amplitude, but of opposite polarity, whereby, for example, a first area has a magnetization of +.phi. and the adjacent area has a magnetization of -.phi.. The bounary between two adjacent magnetic areas which follow one another along a track or zone defines a magnetization sense change ora "magnetic transition".
There are two different types of magnetic transistions, namely: when the magnetic head passes successive magnetic areas having negative and positive induction on the disc, the magnetization sense change is positive; and, when, on the other hand, the head passes successive areas having positive and negative induction, the magnetization sense change is negative.
In the present practice, the preamble data of each reference zone ZRP.sub.ij are recorded at the beginning of the reference zone so that they are chronologically the first data to be read as the zone is translated past the head. Thereby, the preamble data are read before the track location data. Typically, the preamble data consist of plural magnetic transitions which alternately differ in polarity and which follow each other within the reference zone.
In response to the preamble data, the head derives a signal that includes alternate analog pulses of opposite polarity. The analog pulses are coupled to a device for controlling the gain of the amplifier circuits. The device for controlling the gain of the amplifier circuits determines the absolute value of the maximum amplitude of the pulses and compares the absolute value of the maximum amplitude to a reference signal, to derive an error signal which controls the gain of the amplifier circuits. The gain of the amplifier circuits is thus a function of the amplitude and polarity of the error signal.
The prior art gain control device has been found to have a particular disadvantage. In particular, for a track having address j in the interval between a reference zone associated with sector S.sub.i to a reference zone of sector S.sub.i+1, the maximum amplitude of the head output may vary by a relatively large amount relative to a reference level because of many of the reasons specified above, such as a fault in the recording layer or a variation in distance between the gap of the head and the disc surface. The relatively large variation in the maximum amplitude leads to a substantial variation of the gain of the amplifier circuits, and consequently of the amplitude of output signals derived from the amplifier circuits. Because of this factor, the risk of errors in considering the data and determining data values cannot be ignored, with regard to either the track location data contained in zone ZRP.sub.ij or in the following sector containing data to be processed.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for and method of controlling the gain of circuitry responsive to signals read by playback heads from data carriers.
Another object of the invention is to provide a new and improved apparatus for and method of maintaining the amplitude of signals derived from different portions of a magnetic disc memory substantialy constant.
A further object of the present invention is to provide a new and improved apparatus for and method of maintaining the amplitude of signals read by different magnetic heads of a magnetic disc memory substantially constant.
An additional object of the invention is to provide a new and improved apparatus for and method of maintaining the amplitude of signals read from a magnetic disc memory substantially constant, despite variations in the radial position of a head, faults in a layer of the disc memory, and variations in distance between a head gap and a magnetic disc surface.
Still another object of the invention is to provide a new and improved apparatus for and method of maintaining the amplitude of signals read by a head responsive to a magnetic disc substantially constant, even though the maximum amplitude of a signal derived by the head from a reference zone varies by a relatively large amount.